Coal seams contain methane gas. In some cases, this gas is produced in commercial quantities. More often, the gas is not producible in commercial quantities, but is a hazard to mining operations. In such cases, the gas is often partially removed prior to or during mining operations by various known degasification procedures.
Fracturing of the coal seams is sometimes used to enhance the rate of gas production when the gas is to be produced, either commercially or for degasification purposes. However, most coal seams have a series of natural fractures, or cleats, which interfere with hydraulic fracturing processes. The present invention is directed to a procedure for reducing the effects of natural cleats on fracturing processes, and for enhancing the effectiveness of the fracturing processes.
A fracturing process for coal seams using a volatile liquid such as ammonia as the fracturing fluid is described in U.S. Pat. No. 3,384,416. A process involving injection of ammonia into a coal seam to cause the coal seam to shatter followed by blowing gas through the area of shattered coal to carry coal particles from the treated area is described in U.S. Pat. No. 3,815,826.
Conventional fracturing of coal seams involves injecting a fracturing fluid, with or without propping agents and fluid loss additives, under fracturing pressure into the coal seams. The fractures created increase the gas permeability in the treated area and enhance gas production or degasification.
Conventional fracturing of coal seams is of limited effectiveness if the treatment zone includes a natural fracture or cleat, as the cleat provides a path for the fracturing fluid and diminishes the extent of induced fractures. Fluid loss additives and the like are of limited effectiveness.
There has been a need for an improved method of fracturing coal seams which contain natural fractures in the treated area. Such a method is provided by the present invention.